Semiconductor-on-insulator (SOI) devices are widely used in microelectronics. In general, SOI devices include active devices such as transistors in a thin semiconductor layer which is on an insulator. In contrast, bulk semiconductor devices include active devices such as transistors in a bulk semiconductor region. SOI devices often use a layer of monocrystalline silicon as a semiconductor material. These devices are often referred to as silicon-on-insulator devices.
Transistors which are formed using SOI technology, hereinafter referred to as SOI transistors, can provide improved isolation and can generally withstand higher supply voltages than bulk semiconductor devices. Moreover, thin film SOI transistors generally have small subthreshold swings and may be used with operating voltages of two volts or less without degrading the operation thereof.
Unfortunately, SOI transistors may be susceptible to bipolar-induced breakdown due to the floating body thereon.
FIG. 1 is a cross-sectional view which illustrates a conventional SOI transistor, and particularly SOI CMOS transistors. As shown in FIG. 1, an insulating layer 3 is located on a semiconductor substrate 1. A transistor, comprising a gate 7, a source 8 and a drain 9 is formed in a thin semiconductor film on the insulating layer 3. Also included is a body layer 5 where the channels of the transistor are located, between the source 8 and drain 9.
As shown in FIG. 1, unlike conventional bulk transistors, the SOI transistor generally does not include a contact which can apply a voltage to the body layer 5. Thus, the body layer 5 is floating. Unfortunately, the floating body layer 5 may reduce the breakdown voltage of the transistor due to the formation of a parasitic bipolar transistor.
For example, for an N-type transistor, if electrons reach a depletion region of the drain 9 from the source 8 where the electric field of the drain is increased by the increased voltage of the drain 9, impact ionization may occur due to the strong forces caused by the increased electric field. Electron hole pairs may be generated. Electrons are extracted through a drain electrode (not shown in FIG. 1) and holes move toward the body layer 5 and are stored thereat.
Accordingly, the potential of the body layer 5 may increase so that the junction between the body layer 5 and the source is forward-biased. The electrons which are injected from the source 8 to the body layer 5 can create a parasitic bipolar transistor in which the source 8, the body layer 5 and the drain 9 function as an emitter, a base and a collector, respectively.
When the parasitic bipolar transistor is formed, a snap-back phenomena may be produced such that the drain current abruptly increases when reduced voltage is applied to the source and drain of the SOI transistor. Accordingly, the breakdown voltage of the SOI transistor may be reduced.
It will be understood that the floating body effect described above may be reduced by forming contacts to the body layer 5. However, since each body layer 5 is isolated from the remaining transistors in a conventional SOI device, it may be difficult to form body layer contacts. Moreover, for highly integrated devices containing many transistors, the body layer contact for each contact may reduce the integration density of the device.